overlap profits

i do not understand the concept/logic behind why in the first year of trading, the profits are portioned from the date of commencement till the end of tax year.

for ex. if trade has commenced on 1 jan 2019 and prepares the account on June 2020 with profit of 50000. then the profit is apportioned into (3/6)*50000= 25000 to assessed for tax year 2018/19. and again the whole of profit 50000 is assess again in the tax year 19/20.

why not just assess the whole of 50000 in the tax year of 19/20 when received like its done on normal basis. why apportion in this way. im missing the logic behind it